I'd say it depends on how extensive and how new your Canon lens bag is, whether you need autofocus, and whether the A7R's shutter shock is going to be a big issue for you.
If you do need autofocus, remember to factor into the cost of a system switch replacing nearly all your lenses, and Sony FE glass is significantly more expensive than the Canon equivalent. Making no value judgements about how good the lenses are- just saying that replacing a normal-ish bag of 16-35 f/2.8 zoom, 70-210 f/2.8 zoom, and a few primes is going to cost you significantly more than the body and more than the Canon kit you likely already have. So bear the lens cost in mind.
The A7Rii might actually might be a cheaper option if you have new (6 years or less) Canon glass, as it should allow useable autofocus, thus letting you get away without changing all your lenses all at once. In my experience no adapted lens ever "just works" as reliably as putting a native lens on a native body, though.
My Canon lenses are too old to autofocus via Metabones on the A7Rii (and definitely won't on the A7R if I understand correctly) but I don't mind so much for landscape work; for people photography though it is clear that only native lenses will cut the mustard for my professional use anyway. So I'm going to have to invest in a Sony/Zeiss FE 85 or 90 mm and a 35 mm at minimum, and I dare say I'll end up with a 16-35 and 70-210 if I stick with the Sony for very long.
If you can pick up a cheap A7R second hand I'd go for it, but all in all I don't know how much of an improvement you'll see from the 6D anyway. It's more that it has a different set of compromises.
If you've got a sack full of L-series lenses or Canon-mount Zeiss, I'd just buy a 5DS R and be done with it, especially if low-light hand-held shooting isn't your primary activity. The Canon may look a bit dull on paper but the people I know who have one say it does provide a significant step-up from older Canons and is more comparable to the Sony's in real life use cases than the lab tests might lead you to believe.
Cheers, Hywel
